A circuit in a raster output scanner for calibrating a laser's power during the scan. The circuit looks ahead to find particular patterns of OFF and ON pixels, and uses a table look-up to generate reference levels to which the laser output can be compared.
In a typical laser driven raster output scanner (ROS), the laser can be calibrated by measuring the laser power before the regular scan starts, and comparing that measurement to a predetermined reference to control the laser power at a desired power level. However, during the scan, typically after a series of ON or OFF pixels, the laser temperature or power may drift from its rated values. This fluctuation of power results in decreased image quality, and could possibly result in damage to the laser. As a result, during the scan, the laser power in some models is slightly decreased as a function of time since the last calibration to reduce the possibility of damage, with a corresponding decrease of image quality.
A considerable improvement would be gained if the laser were calibrated more frequently, which would necessarily mean that the laser would have to be calibrated during the active portion of the scan. A problem is that the calibration process must measure the power over a number of pixels, and the data during the scan usually is an irregular collection of ON and OFF pulses, which would seem to make any calibration inaccurate. A method for accurately calibrating the laser during the active scan period is needed.